Observation optical system and optical apparatus having the same

ABSTRACT

An observation optical system includes, in order from an object side, an objective lens having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, an erect optical system, and an eyepiece lens of positive refractive power for observing an object image formed through the objective lens, wherein an air separation between the first lens unit and the second lens unit and an air separation between the second lens unit and the third lens unit are made variable to effect variation of magnification.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to observation optical systems such as telescopes and binoculars and, more particularly, to an observation optical system having the zoom function of varying the magnification of an image being observed.

[0003] 2. Description of Related Art

[0004] The main stream of conventional zoom binoculars is that zooming is performed by a variable magnification eyepiece lens having a moving lens unit. FIG. 36 is a schematic diagram showing one example of the conventional zoom binoculars, in which, for the purpose of simplicity, only one of the right and left optical systems of the binocular is illustrated.

[0005] In FIG. 36, reference numeral 100 denotes an objective lens of positive refractive power. An erect prism P is provided for erecting, laterally reversing an object image OBI formed mainly by the objective lens 100. A variable magnification eyepiece lens 200 comprises a first lens unit 201 of negative refractive power, a second lens unit 202 of positive refractive power and a third lens unit 203 of positive refractive power. The object image OBI is formed at a position before (on the object side of) the second lens unit 202 through the objective lens 100, the erect prism P and the first lens unit 201 of the eyepiece lens 200.

[0006] In the binocular of such construction, the first lens unit 201 of the variable magnification eyepiece lens 200 is made axially movable for varying the image magnification, and to compensate for the image shift, or to correct the diopter, the second lens unit 202 is made to axially move.

[0007] Such a variable magnification eyepiece lens 200 is favorable to realize zoom binoculars relatively easily. Up to now, therefore, this arrangement has been very popularly employed.

[0008] However, the use of the above-described construction and arrangement for the variable magnification eyepiece lens in the zoom binoculars results in as narrow an apparent angle of view as 35-40 degrees at the low magnifications. To widen the field coverage, there is a need to increase the diameter of the second lens unit 202 and the bulk of the erect prism P. For this reason, it is difficult for the prior art arrangement to simultaneously fulfill the requirements of increasing the apparent field angle and of minimizing the bulk and size of the binocular.

BRIEF SUMMARY OF THE INVENTION

[0009] An object of the invention is to provide an observation optical system capable of increasing the field angle at the low magnification (in the wide-angle end) without having to increase the bulk and size of an apparatus having the observation optical system.

[0010] To attain the above object, in accordance with a first aspect of the invention, there is provided an observation optical system, which comprises, in order from an object side, an objective lens having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, an erect optical system, and an eyepiece lens of positive refractive power for observing an object image formed through the objective lens, wherein an air separation between the first lens unit and the second lens unit and an air separation between the second lens unit and the third lens unit are made variable to effect variation of magnification.

[0011] In accordance with a second aspect of the invention, there is provided an observation optical system, which comprises, in order from an object side, an objective lens having a front lens unit of positive refractive power, a variator lens unit and a compensator lens unit, an erect optical system, and an eyepiece lens of positive refractive power for observing an object image formed through the objective lens, wherein the following condition is satisfied:

0.7<βt<1.4

[0012] where βt is an overall lateral magnification in a telephoto end of the objective lens excluding the front lens unit.

[0013] In accordance with a third aspect of the invention, there is provided an optical apparatus comprising the observation optical system according to one of the first and second aspects described above.

[0014] In accordance with a fourth aspect of the invention, there is provided a binocular, which comprises, in order from an object side, a pair of objective lenses each having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, a pair of erect optical systems, and a pair of eyepiece lenses of positive refractive power for observing a pair of object images respectively formed through the pair of objective lenses, wherein in each of the pair of objective lenses, an air separation between the first lens unit and the second lens unit and an air separation between the second lens unit and the third lens unit are made variable to effect variation of magnification.

[0015] In accordance with a fifth aspect of the invention, there is provided a binocular, which comprises, in order from an object side, a pair of objective lenses of variable focal length, a pair of erect optical systems each having an entering optical axis and an exiting optical axis deviating from each other, and a pair of eyepiece lenses, wherein interpupillary adjustment is performed by rotating the erect optical systems and the eyepiece lenses around respective optical axes, as rotation axes, of the objective lenses.

[0016] The above and further objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

[0017]FIG. 1 is a longitudinal section view of an observation optical system according to a first embodiment of the invention with the eyepiece lens not shown.

[0018]FIGS. 2A to 2D are graphic representations of the aberrations of the objective lens of the first embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0019]FIGS. 3A to 3D are graphic representations of the aberrations of the objective lens of the first embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0020]FIGS. 4A to 4D are graphic representations of the aberrations of the objective lens of the first embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0021]FIGS. 5A to 5D are graphic representations of the lateral aberrations with the axial beam of the objective lens of the first embodiment in the wide-angle end when a respective one of the lens units is moved in a direction perpendicular to an optical axis.

[0022]FIGS. 6A to 6D are graphic representations of the lateral aberrations with the axial beam of the objective lens of the first embodiment in the telephoto end when a respective one of the lens units is moved in a direction perpendicular to an optical axis.

[0023]FIG. 7 is a longitudinal section view of a binocular according to a second embodiment of the invention.

[0024]FIGS. 8A to 8D are graphic representations of the aberrations with the afocal beam of the binocular of the second embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0025]FIGS. 9A to 9D are graphic representations of the aberrations with the afocal beam of the binocular of the second embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0026]FIGS. 10A to 10D are graphic representations of the aberrations with the afocal beam of the binocular of the second embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0027]FIG. 11 is a longitudinal section view of a binocular according to a third embodiment of the invention.

[0028]FIGS. 12A to 12D are graphic representations of the aberrations with the afocal beam of the binocular of the third embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0029]FIGS. 13A to 13D are graphic representations of the aberrations with the afocal beam of the binocular of the third embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0030]FIGS. 14A to 14D are graphic representations of the aberrations with the afocal beam of the binocular of the third embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0031]FIG. 15 is a longitudinal section view of a binocular according to a fourth embodiment of the invention.

[0032]FIGS. 16A to 16D are graphic representations of the aberrations with the afocal beam of the binocular of the fourth embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0033]FIGS. 17A to 17D are graphic representations of the aberrations with the afocal beam of the binocular of the fourth embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0034]FIGS. 18A to 18D are graphic representations of the aberrations with the afocal beam of the binocular of the fourth embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0035]FIG. 19 is a longitudinal section view of an observation optical system according to a fifth embodiment of the invention.

[0036]FIGS. 20A to 20D are graphic representations of the aberrations of the objective lens of the fifth embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0037]FIGS. 21A to 21D are graphic representations of the aberrations of the objective lens of the fifth embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0038]FIGS. 22A to 22D are graphic representations of the aberrations of the objective lens of the fifth embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0039]FIGS. 23A to 23D are graphic representations of the lateral aberrations with the axial beam of the fifth embodiment when the variable angle prism VAP is rendered operative.

[0040]FIG. 24 is a longitudinal section view of an observation optical system according to a sixth embodiment of the invention.

[0041]FIGS. 25A to 25D are graphic representations of the aberrations of the binocular of the sixth embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0042]FIGS. 26A to 26D are graphic representations of the aberrations of the binocular of the sixth embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0043]FIGS. 27A to 27D are graphic representations of the aberrations of the binocular of the sixth embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0044]FIGS. 28A to 28D are graphic representations of the lateral aberrations with the axial beam of the sixth embodiment when the variable angle prism VAP is rendered operative.

[0045]FIG. 29 is a longitudinal section view of an observation optical system according to a seventh embodiment of the invention.

[0046]FIGS. 30A to 30D are graphic representations of the aberrations of the binocular of the seventh embodiment in the wide-angle end when the image-shake compensating function is rendered inoperative.

[0047]FIGS. 31A to 31D are graphic representations of the aberrations of the binocular of the seventh embodiment in a middle focal length position when the image-shake compensating function is rendered inoperative.

[0048]FIGS. 32A to 32D are graphic representations of the aberrations of the binocular of the seventh embodiment in the telephoto end when the image-shake compensating function is rendered inoperative.

[0049]FIGS. 33A to 33D are graphic representations of the lateral aberrations with the axial beam of the seventh embodiment when the variable angle prism VAP is rendered operative.

[0050]FIGS. 34A and 34B are respectively a top view and a back view of a binocular according to an eighth embodiment of the invention.

[0051]FIGS. 35A and 35B are respectively a side view and a back view of a binocular according to a ninth embodiment of the invention.

[0052]FIG. 36 is a schematic diagram of a thin lens system in the conventional binocular.

DETAILED DESCRIPTION OF THE INVENTION

[0053] Hereinafter, preferred embodiments of the invention will be described in detail with reference to the drawings.

(First Embodiment)

[0054]FIG. 1 in block diagram shows an objective lens and an erect prism according to a first embodiment of the invention, as applied to an observation optical system such as a binocular or a telescope. In FIG. 1, for the purpose of simplicity, the eyepiece lens is not illustrated here, but what is suitable as the eyepiece lens to the objective lens of the first embodiment will become apparent in the other embodiments to be described later.

[0055] Referring to FIG. 1, the objective lens is constructed with three lens units 1 to 3, i.e., in order from an object side, a first lens unit 1 of positive refractive power, a second lens unit 2 of negative refractive power and a third lens unit 3 of positive refractive power. During zooming from the wide-angle end to the telephoto end, the first lens unit 1 remains stationary, the second lens unit 2 axially moves from the object side to the observation side, and the third lens unit 3 axially moves from the observation side to the object side in such relation as to compensate for the shift of an image plane resulting from the movement of the second lens unit 2. The erect prism P is illustrated in developed form, which has the function of erecting and laterally reversing an object image OBI formed by the objective lens. Then, the object image OBI formed by the objective lens is enlarged by an eyepiece lens (not shown) and is guided to the observer's eye.

[0056] The observation optical system shown in FIG. 1 has an image stabilizing function of preventing the shake of an image plane caused by the vibration of the hands holding the apparatus. The image stabilizing function is performed in such a manner that, based on information on the vibration detected by a vibration gyro-sensor (not shown), at least one of the first lens unit 1, the second lens unit 2 and the third lens unit 3 is made to displace in directions perpendicular to an optical axis.

[0057] In a case where the first lens unit 1 is made to displace in directions perpendicular to the optical axis to attain the image stabilizing function, the required amount of movement of the first lens unit 1 for the vibration information can be made constant irrespective of the change of the zooming position, so that there is an advantage of assuring simplification of a control circuit (not shown) for controlling the movement of the first lens unit 1.

[0058] On the other hand, in a case where the second unit 3 axially moves from the observation side to the object side in such relation as to compensate for the shift of an image plane resulting from the movement of the second lens unit 2. The erect prism P is illustrated in developed form, which has the function of erecting and laterally reversing an object image OBI formed by the objective lens. Then, the object image OBI formed by the objective lens is enlarged by an eyepiece lens (not shown) and is guided to the observer's eye.

[0059] The observation optical system shown in FIG. 1 has an image stabilizing function of preventing the shake of an image plane caused by the vibration of the hands holding the apparatus. The image stabilizing function is performed in such a manner that, based on information on the vibration detected by a vibration gyro-sensor (not shown), at least one of the first lens unit 1, the second lens unit 2 and the third lens unit 3 is made to displace in directions perpendicular to an optical axis.

[0060] In a case where the first lens unit 1 is made to displace in directions perpendicular to the optical axis to attain the image stabilizing function, the required amount of movement of the first lens unit 1 for the vibration information can be made constant irrespective of the change of the zooming position, so that there is an advantage of assuring simplification of a control circuit (not shown) for controlling the movement of the first lens unit 1.

[0061] On the other hand, in a case where the second lens unit 2 or the third lens unit 3 is made to displace in directions perpendicular to the optical axis to attain the image stabilizing function, the lens unit to be moved is relatively small in diameter and light in weight, so that there is an advantage that an actuator of small size suffices for driving that lens unit.

[0062] In the first embodiment as applied to the binocular, focusing is carried out by axially moving the first lens unit 1. The axial alignment between the left and right optical systems is adjusted in the reference position by moving the first lens unit 1 in a direction perpendicular to the optical axis.

[0063]FIGS. 2A to 2D through FIGS. 4A to 4D show the aberrations of the objective lens of the first embodiment (numerical example 1) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function (image stabilizing function) is rendered inoperative.

[0064]FIGS. 5A to 5D show the comparison of the lateral aberrations with the axial beam of the first embodiment in the wide-angle end when a respective one of the lens units is moved in a direction perpendicular to the optical axis until the angle of deviation of ray on the object side reaches 0.3 degrees. FIG. 5A shows a case in the normal position (with all the lens units in a common axis). FIGS. 5B to 5D show cases when a respective one of the first to third lens units 1 to 3 is made to move in a direction perpendicular to the optical axis.

[0065]FIGS. 6A to 6D show the comparison of the lateral aberrations with the axial beam of the first embodiment in the telephoto end when a respective one of the lens units is moved in a direction perpendicular to the optical axis until the angle of deviation of ray on the object side reaches 0.3 degrees. FIG. 6A shows a case in the normal position (with all the lens units in a common axis). FIGS. 6B to 6D show cases when a respective one of the first to third lens units 1 to 3 is made to move in a direction perpendicular to the optical axis.

[0066] The use of the objective lens of the first embodiment in the observation optical system leads to a possibility that even if any one of the lens units is parallel-decentered from the optical axis, the lateral aberration changes to very little extent, as shown in FIGS. 5A to 5D and FIGS. 6A to 6D. Accordingly, a good optical performance can be obtained even when the image-shake compensating function is rendered operative.

(Second Embodiment)

[0067]FIG. 7 is a lens block diagram of a binocular according to a second embodiment of the invention. For the purpose of simplicity, in FIG. 7, there is illustrated only one of the left and right lens systems of the binocular. The second embodiment shows an example of the eyepiece lens suited to the objective lens of the first embodiment. A first lens unit 1, a second lens unit 2 and a third lens unit 3 which constitute an objective lens are exactly the same as those shown in the first embodiment. The eyepiece lens 4 is arranged to enlarge an object image OBI formed on the observation side of the erect prism P, while guiding the light beam therefrom to the observer's eye.

[0068]FIGS. 8A to 8D through FIGS. 10A to 10D show the aberrations with the afocal light beam of the binocular of the second embodiment (numerical example 2) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function (image stabilizing function) is rendered inoperative.

(Third Embodiment)

[0069]FIG. 11 is a lens block diagram of a binocular according to a third embodiment of the invention.

[0070] The third embodiment shows another example of the eyepiece lens suited to the objective lens of the first embodiment. The eyepiece lens 4 of the third embodiment is different in construction and arrangement from that of the second embodiment, including a field flattener lens 4A as disposed on the object side of the object image OBI formed by the objective lens and the erect prism P. By using the field flattener lens 4A, the binocular of the third embodiment is better corrected particularly for curvature of field and astigmatism than that of the second embodiment.

[0071]FIGS. 12A to 12D through FIGS. 14A to 14D show the aberrations with the afocal light beam of the binocular of the third embodiment (numerical example 3) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function (image stabilizing function) is rendered inoperative.

(Fourth Embodiment)

[0072]FIG. 15 is a lens block diagram of a binocular according to a fourth embodiment of the invention.

[0073] The fourth embodiment shows yet another example of the eyepiece lens suited to the objective lens of the first embodiment. The eyepiece lens 4 of the fourth embodiment is different in construction and arrangement from that of the second embodiment, including a field flattener lens 4A as disposed on the object side of the object image OBI formed by the objective lens and the erect prism P. By using the field flattener lens 4A and increasing the number of those of the constituent lenses which lie on the observation side of the object image OBI, the binocular of the third embodiment has a longer eye relief and is better corrected for curvature of field and astigmatism than that of the second embodiment.

[0074]FIGS. 16A to 16D through FIGS. 18A to 18D show the aberrations with the afocal light beam of the binocular of the fourth embodiment (numerical example 4) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function (image stabilizing function) is rendered inoperative.

(Fifth Embodiment)

[0075]FIG. 19 is a longitudinal section view showing the objective lens and the erect prism P in the binocular according to a fifth embodiment of the invention. In the fifth embodiment, the objective lens composed of first to third lens units 1 to 3 is the same as that of the first embodiment. For the purpose of simplicity, FIG. 19 is drawn by omitting an eyepiece lens, but the eyepiece lens shown in any of the second to fourth embodiments is used as a suitable one.

[0076] A characteristic feature of the fifth embodiment is that a variable angle prism VAP is disposed on the object side of the first lens unit 1, and on the basis of the vibration information detected by a vibration gyro-sensor (not shown), the apex angle of the variable angle prism VAP is varied to compensate for the image shake caused by the vibration of the hands holding the binocular. By arranging the variable angle prism VAP in the frontmost position as in the fifth embodiment, the amount of the varied apex angle for the equivalent amplitude of vibration can be made constant in any zooming position, thus assuring simplification of an electric circuit (not shown) for controlling the variable angle prism VAP.

[0077]FIGS. 20A to 20D through FIGS. 22A to 22D show the aberrations of the objective lens of the fifth embodiment (numerical example 5) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function is rendered inoperative.

[0078]FIGS. 23A to 23D show the comparison of the lateral aberrations with the axial light beam of the lens system of the fifth embodiment between when in the normal state and when the variable angle prism VAP is operated to deflect the principal ray to 0.3 degrees on the object side. FIG. 23A shows a case in the wide-angle end when in the normal state where the rays of light are not deflected by the variable angle prism VAP. FIG. 23B shows a case in the wide-angle end when the variable angle prism VAP is operative. FIG. 23C shows a case in the telephoto end when in the normal state. FIG. 23D shows a case in the telephoto end when the variable angle prism VAP is operative.

[0079] With the variable angle prism VAP operating in the telephoto end, somewhat decentering lateral chromatic aberration is produced as shown in FIG. 23D. In the other positions, however, almost no decentering aberrations are produced. A good optical performance can thus be obtained even when the image-shake compensating function is rendered operative.

(Sixth Embodiment)

[0080]FIG. 24 is a longitudinal section view showing the objective lens and the erect prism P in the binocular according to a sixth embodiment of the invention. In the sixth embodiment, too, the objective lens composed of first to third lens units 1 to 3 is the same as that of the first embodiment. Also, for the purpose of simplicity, FIG. 24 is drawn by omitting an eyepiece lens, but the eyepiece lens shown in any of the second to fourth embodiments is used as a suitable one.

[0081] A characteristic feature of the sixth embodiment is that a variable angle prism VAP is disposed in a space between the first lens unit 1 and the second lens unit 2, and on the basis of the vibration information detected by a vibration gyro-sensor (not shown), the apex angle of the variable angle prism VAP is varied to compensate for the image shake caused by the vibration of the hands holding the binocular. By arranging the variable angle prism VAP in the interior of the objective lens as in the sixth embodiment, the variable angle prism to be used can be reduced in the size, thus assuring minimization of the bulk and size of the apparatus as a whole.

[0082]FIGS. 25A to 25D through FIGS. 27A to 27D show the aberrations of the objective lens of the sixth embodiment (numerical example 6) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function is rendered inoperative.

[0083]FIGS. 28A to 28D show the comparison of the lateral aberrations with the axial light beam of the lens system of the sixth embodiment between when in the normal state and when the variable angle prism VAP is operated to deflect the principal ray to 0.3 degrees on the object side. FIG. 28A shows a case in the wide-angle end when in the normal state where the rays of light are not deflected by the variable angle prism VAP. FIG. 28B shows a case in the wide-angle end when the variable angle prism VAP is operative. FIG. 28C shows a case in the telephoto end when in the normal state. FIG. 28D shows a case in the telephoto end when the variable angle prism VAP is operative.

[0084] With the variable angle prism VAP operating in the telephoto end, somewhat decentering lateral chromatic aberration is produced as shown in FIG. 28D. In the other positions, however, almost no decentering aberrations are produced. A good optical performance can thus be obtained even when the image-shake compensating function is rendered operative.

(Seventh Embodiment)

[0085]FIG. 29 is a longitudinal section view showing the objective lens and the erect prism P in the binocular according to a seventh embodiment of the invention. In the seventh embodiment, too, the objective lens composed of first to third lens units 1 to 3 is the same as that of the first embodiment. Also, for the purpose of simplicity, FIG. 29 is drawn by omitting an eyepiece lens, but the eyepiece lens shown in any of the second to fourth embodiments is used as a suitable one.

[0086] A characteristic feature of the seventh embodiment is that a variable angle prism VAP is disposed in a space between the third lens unit 3 and the erect prism P, and on the basis of the vibration information detected by a vibration gyro-sensor (not shown), the apex angle of the variable angle prism VAP is varied to compensate for the image shake caused by the vibration of the hands holding the binocular. By arranging the variable angle prism VAP in the interior of the lens system as in the seventh embodiment, the variable angle prism to be used can be reduced in the size, thus assuring minimization of the bulk and size of the apparatus as a whole.

[0087]FIGS. 30A to 30D through FIGS. 32A to 32D show the aberrations of the objective lens of the seventh embodiment (numerical example 7) in the wide-angle end, a middle focal length position and the telephoto end, respectively, when the image-shake compensating function is rendered inoperative.

[0088]FIGS. 33A to 33D show the comparison of the lateral aberrations with the axial light beam of the lens system of the seventh embodiment between when in the normal state and when the variable angle prism VAP is operated to deflect the principal ray to 0.3 degrees on the object side. FIG. 33A shows a case in the wide-angle end when in the normal state where the rays of light are not deflected by the variable angle prism VAP. FIG. 33B shows a case in the wide-angle end when the variable angle prism VAP is operative. FIG. 33C shows a case in the telephoto end when in the normal state. FIG. 33D shows a case in the telephoto end when the variable angle prism VAP is operative.

[0089] With the variable angle prism VAP operating in the telephoto end, somewhat decentering lateral chromatic aberration is produced as shown in FIG. 33D. In the other positions, however, there are almost no decentering aberrations. A good optical performance can thus be obtained even when the image-shake compensating function is rendered operative.

(Eighth Embodiment)

[0090]FIGS. 34A and 34B are respectively a top view and a back view (as looked from the observer) of a binocular according to an eighth embodiment of the invention in which, as the erect prism of the second embodiment, use is made of a large rectangular prism having adhered thereto two small rectangular prisms in differently oriented relation to form a porro-II type prism.

[0091] In the binocular of the eighth embodiment, a pair of the porro-II type prisms PL and PR are arranged upon interpupillary adjustment to bring the apices A and B of the large rectangular prisms into confrontation at a middle point in the adjustment width as shown in FIG. 34B. The left and right porro-II prisms PL and PR and the eyepiece lenses 4 are rotated respectively around the optical axes OAL and OAR of the left and right objective lenses to adjust the interpupillary distance ID to the individual users.

[0092] Such an arrangement of the porro-II prisms PL and PR enables a large width of interpupillary adjustment to be realized by a small angle of rotation. Moreover, the prism casings do not awkwardly extrude, thus making it possible to realize a compact zoom binocular as a whole. Also, since the eyepiece lenses 4 in the pair are able to axially move independently of each other, the left and right eyesight can be adjusted and the position error of the plane of sharp focus resulting from zooming can be corrected.

(Ninth Embodiment)

[0093]FIGS. 35A and 35B are respectively a side view (as looked from the left) and a back view of a binocular according to a ninth embodiment of the invention in which, as the erect prism of the second embodiment, use is made of a combination of an auxiliary or front prism FPL (FPR) and a roof prism DPL (DPR) each having an entering optical axis and an exiting optical axis deviating from each other.

[0094] In the binocular of the ninth embodiment, a pair of the roof prisms DPL and DPR are arranged upon interpupillary adjustment to bring their apices C and D into upward orientation relative to the left and right objective lenses at a middle point in the adjustment width as shown in FIG. 35B. The left and right assemblies of auxiliary prisms FPL and FPR, roof prisms DPL and DPR and eyepiece lenses 4 are rotated respectively around the optical axes OAL and OAR of the left and right objective lenses to adjust the interpupillary distance ID to the individual users.

[0095] Such an arrangement of the auxiliary prism FPL (FPR) and roof prism DPL (DPR) enables a large width of interpupillary adjustment to be realized by a small angle of rotation. Moreover, the prism casings do not awkwardly extrude, thus making it possible to realize a compact zoom binocular as a whole.

[0096] Next, on the design of the observation optical system of the invention, preferable conditions are described below.

[0097] Letting the overall lateral magnification in the telephoto end of all the lens units excluding the first lens unit in the objective lens be denoted by βt, it is preferred that the following condition is satisfied:

0.7<βt<1.4   (1)

[0098] Further, the range of the above-described condition (1) is more desirably altered as follows:

0.8<βt<1.3   (1′)

[0099] In the ranges beyond the lower and upper limits of the condition (1), the overall powers of all the lens units excluding the first lens unit becomes too strong in the telephoto end. In a case where the optical axis of the objective lens is slightly inclined, the variation with zooming of the left and right optical axes becomes objectionably large. Here, on the assumption that the overall lateral magnification in the telephoto end of the second and third lens units is “1”, then the overall power is lost in the telephoto end. In a case where the optical axes of the left and right objective lenses are found to have inclined, the first lens unit is moved in a direction perpendicular to the optical axis, thus adjusting the left and right axial alignments. Then, even when zooming to the telephoto end is performed, no zooming error occurs between the left and right optical axes.

[0100] In the first to ninth embodiments, the overall lateral magnification of the objective lens excluding the first lens unit, that is, the overall lateral magnification βt of the second and third lens units in the telephoto end is “0.910”.

[0101] Also, for the binocular of the type shown in the ninth embodiment, it is preferred to satisfy the following condition:

45°<A<135°  (2)

[0102] where A is an angle a line which connects optical axes of the objective lenses with each other makes with respect to a line which connects the optical axis of each of the objective lenses with an apex of a roof surface of each of the roof prisms in a middle position in a width of adjustment for interpupillary adjustment.

[0103] Further, the range of the above-described condition (2) is more desirably altered as follows:

55°<A<125°  (2′)

[0104] In the ranges beyond the lower and upper limits of the condition (2), even if the auxiliary prism, roof prism and eyepiece lens are rotated around the optical axis of the objective lens to any angle, the interpupillary distance does not result in large enough change. So, these violations are objectionable.

[0105] In the ninth embodiment, the angle A is as follows:

A=90°.

[0106] Next, seven numerical examples 1 to 7 corresponding to the first to seventh embodiments described above are shown. In the numerical data for the examples 1 to 7, ri is the radius of curvature of the i-th lens surface, when counted from the object side, di is the separation between the i-th and (i+1)st surfaces, when counted from the object side, ni is the refractive index for the spectral d-line of the i-th optical member, when counted from the object side, and υi is the Abbe number of the i-th optical member, when counted from the object side. (Numerical Example 1) f = 61.55˜175.17 Fno = 1:4.3˜4.6 2ω = 13°˜4.6° r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = −78.491 d2 = 1.90 n2 = 1.60342 ν2 = 38.0 r3 = −348.351 d3 = Variable r4 = −89.765 d4 = 1.20 n3 = 1.72342 ν3 = 38.0 r5 = 23.994 d5 = 4.43 n4 = 1.84666 ν4 = 23.8 r6 = 60.816 d6 = Variable r7 = 54.304 d7 = 4.05 n5 = 1.72342 ν5 = 38.0 r8 = −214.784 d8 = 3.48 r9 = 28.962 d9 = 2.00 n6 = 1.84666 ν6 = 23.8 r10 = 17.740 d10 = 5.69 n7 = 1.48749 ν7 = 70.2 r11 = 48.670 d11 = Variable r12 = ∞ d12 = 80.00 n8 = 1.56833 ν8 = 56.1 r13 = ∞ Variable Focal Length Separation 61.55 107.33 175.17 d3 25.38 50.92 76.47 d6 65.47 30.56 1.62 d11 10.67 20.03 23.42 (Numerical Example 2) r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = −78.491 d2 = 1.90 n2 = 1.60342 ν2 = 38.0 r3 = −348.351 d3 = Variable r4 = 89.765 d4 = 1.20 n3 = 1.72342 ν3 = 38.0 r5 = 23.994 d5 = 4.43 n4 = 1.84666 ν4 = 23.8 r6 = 60.816 d6 = Variable r7 = 54.304 d7 = 4.05 n5 = 1.72342 ν5 = 38.0 r8 = −214.784 d8 = 3.48 r9 = 28.962 d9 = 2.00 n6 = 1.84666 ν6 = 23.8 r10 = 17.740 d10 = 5.69 n7 = 1.48749 ν7 = 70.2 r11 = 48.670 d11 = Variable r12 = ∞ d12 = 80.00 n8 = 1.56883 ν8 = 56.3 r13 = ∞ d13 = 19.25 r14 = −30.945 d14 = 1.00 n9 = 1.84666 ν9 = 23.8 r15 = 26.111 d15 = 0.15 r16 = 25.422 d16 = 6.21 n10 = 1.60311 ν10 = 60.7 r17 = −11.619 d17 = 0.20 r18 = 24.490 d18 = 3.55 n11 = 1.60311 ν11 = 60.7 r19 = −123.371 d19 = 0.20 r20 = 11.207 d20 = 3.29 n12 = 1.60311 ν12 = 60.7 r21 = 17.951 d21 = 10.00 r22 = ∞ Variable Magnification Separation 5.33× 9.30× 15.18× d3 25.37 50.92 76.47 d6 65.47 30.56 1.62 d11 8.67 18.03 21.42 (Numerical Example 3) r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = −78.491 d2 = 1.90 n2 = 1.60342 ν2 = 38.0 r3 = −348.351 d3 = Variable r4 = −89.765 d4 = 1.20 n3 = 1.72342 ν3 = 38.0 r5 = 23.994 d5 = 4.43 n4 = 1.84666 ν4 = 23.8 r6 = 60.816 d6 = Variable r7 = 54.304 d7 = 4.05 n5 = 1.72342 ν5 = 38.0 r8 = −214.784 d8 = 3.48 r9 = 28.962 d9 = 2.00 n6 = 1.84666 ν6 = 23.8 r10 = 17.740 d10 = 5.69 n7 = 1.48749 ν7 = 70.2 r11 = 48.670 d11 = Variable r12 = ∞ d12 = 80.00 n8 = 1.56883 ν8 = 56.3 r13 = ∞ d13 = 7.22 r14 = −7.887 d14 = 1.08 n9 = 1.51742 ν9 = 52.4 r15 = 39.802 d15 = 1.64 r16 = −33.532 d16 = 5.09 n10 = 1.83400 ν10 = 37.2 r17 = −10.601 d17 = 12.50 r18 = −577.539 d18 = 1.35 n11 = 1.80518 ν11 = 25.4 r19 = 19.330 d19 = 9.43 n12 = 1.62299 ν12 = 58.2 r20 = 16.915 d20 = 2.70 r21 = 16.880 d21 = 5.26 n13 = 1.62299 ν13 = 58.2 r22 = 662.770 d22 = 40.80 r23 = ∞ Variable Magnification Separation 5.66× 9.87× 16.11× d3 25.37 50.92 76.47 d6 65.47 30.56 1.62 d11 8.67 18.03 21.42 (Numerical Example 4) r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = 78.491 d2 = 1.90 n2 = 1.60342 ν2 = 38.0 r3 = −348.351 d3 = Variable r4 = −89.765 d4 = 1.20 n3 = 1.72342 ν3 = 38.0 r5 = 23.994 d5 = 4.43 n4 = 1.84666 ν4 = 23.8 r6 = 60.816 d6 = Variable r7 = 54.304 d7 = 4.05 n5 = 1.72342 ν5 = 38.0 r8 = −214.784 d8 = 3.48 r9 = 28.962 d9 = 2.00 n6 = 1.84666 ν6 = 23.8 r10 = 17.740 d10 = 5.69 n7 = 1.48749 ν7 = 70.2 r11 = 48.670 d11 = Variable r12 = ∞ d12 = 80.00 n8 = 1.56883 ν8 = 56.3 r13 = ∞ d13 = 5.5 r14 = −11.155 d14 = 1.20 n9 = 1.51742 ν9 = 52.4 r15 = 78.480 d15 = 1.88 r16 = −20.766 d16 = 4.00 n10 = 1.83400 ν10 = 37.2 r17 = −11.973 d17 = 12.50 r18 = −23.270 d18 = 1.50 n11 = 1.80518 ν11 = 25.4 r19 = 35.239 d19 = 12.40 n12 = 1.62299 ν12 = 58.2 r20 = −19.307 d20 = 0.20 r21 = 44.900 d21 = 7.75 n13 = 1.62299 ν13 = 58.2 r22 = −44.900 d22 = 0.20 r23 = 23.968 d23 = 1.50 n14 = 1.72825 ν14 = 28.5 r24 = 13.424 d24 = 7.70 n15 = 1.62299 ν15 = 58.2 r25 = 78.226 d25 = 15.00 r26 = ∞ Variable Magnification Separation 5.13× 8.95× 14.61× d3 25.37 50.92 76.47 d6 65.47 30.56 1.62 d11 8.67 18.03 21.42 (Numerical Example 5) f = 61.54585 Fno = 1:4.3˜4.6 2ω = 13°˜4.6° r1 = ∞ d1 = 2.00 n1 = 1.51633 ν1 = 64.2 r2 = ∞ d2 = 4.00 n2 = 1.41650 ν2 = 52.2 r3 = ∞ d3 = 2.00 n3 = 1.51633 ν3 = 64.2 r4 = ∞ d4 = 2.00 r5 = 112.526 d5 = 5.98 n4 = 1.51633 ν4 = 64.2 r6 = −78.491 d6 = 1.90 n5 = 1.60342 ν5 = 38.0 r7 = −348.351 d7 = Variable r8 = −89.765 d8 = 1.20 n6 = 1.72342 ν6 = 38.0 r9 = 23.994 d9 = 4.43 n7 = 1.84666 ν7 = 23.8 r10 = 60.816 d10 = Variable r11 = 54.304 d11 = 4.05 n8 = 1.72342 ν8 = 38.0 r12 = −214.784 d12 = 3.48 r13 = 28.962 d13 = 2.00 n9 = 1.84666 ν9 = 23.8 r14 = 17.740 d14 = 5.69 n10 = 1.48749 ν10 = 70.2 r15 = 48.670 d15 = Variable r16 = ∞ d16 = 80.00 n11 = 1.56883 ν11 = 56.1 r17 = ∞ Variable Focal Length Separation 61.55 107.33 175.17 d3 25.38 50.92 76.47 d6 65.47 30.56 1.62 d11 10.67 20.03 23.42 (Numerical Example 6) f = 61.54577 Fno = 1:4.3˜4.6 2ω = 13°˜4.6° r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = −78.491 d2 = 1.90 n2- 1.60342 ν2 = 38.0 r3 = −348.351 d3 = 17.00 r4 = ∞ d4 = 2.00 n3 = 1.51633 ν3 = 64.2 r5 = ∞ d5 = 4.00 n4 = 1.41650 ν4 = 52.2 r6 = ∞ d6 = 2.00 n5 = 1.51633 ν5 = 64.2 r7 = ∞ d7 = Variable r8 = −89.765 d8 = 1.20 n6 = 1.72342 ν6 = 38.0 r9 = 23.994 d9 = 4.43 n7 = 1.84666 ν7 = 23.8 r10 = 60.816 d10 = Variable r11 = 54.304 d11 = 4.05 n8 = 1.72342 ν8 = 38.0 r12 = −214.784 d12 = 3.48 r13 = 28.962 d13 = 2.00 n9 = 1.84666 ν9 = 23.8 r14 = 17.740 d14 = 5.69 n10 = 1.48749 ν10 = 70.2 r15 = 48.670 d15 = Variable r16 = ∞ d16 = 80.00 n11 = 1.56883 ν11 = 56.1 r17 = ∞ Variable Focal Length Separation 61.55 107.33 175.17 d7  2.91 28.46 54.01 d10 65.47 30.56 1.62 d15 10.67 20.03 23.42 (Numerical Example 7) f = 61.54585 Fno = 1:4.3˜4.6 2ω = 13°˜4.6° r1 = 112.526 d1 = 5.98 n1 = 1.51633 ν1 = 64.2 r2 = −78.491 d2 = 1.90 n2 = 1.60342 ν2 = 38.0 r3 = −348.351 d3 = Variable r4 = −89.765 d4 = 1.20 n3 = 1.72342 ν3 = 38.0 r5 = 23.994 d5 = 4.43 n4 = 1.84666 ν4 = 23.8 r6 = 60.816 d6 = Variable r7 = 54.304 d7 = 4.05 n5 = 1.72342 ν5 = 38.0 r8 = −214.784 d8 = 3.48 r9 = 28.962 d9 = 2.00 n6 = 1.84666 ν6 = 23.8 r10 = 17.740 d10 = 5.69 n7 = 1.48749 ν7 = 70.2 r11 = 48.670 d11 = Variable r12 = ∞ d12 = 1.00 n8 = 1.51633 ν8 = 64.2 r13 = ∞ d13 = 3.00 n9 = 1.41650 ν9 = 52.2 r14 = ∞ d14 = 1.00 n10 = 1.51633 ν10 = 64.2 r15 = ∞ d15 = 3.00 r16 = ∞ d16 = 80.00 n11 = 1.56883 ν11 = 56.1 r17 = ∞ Variable Focal Length Separation 61.55 107.33 175.17 d3 25.38 50.92 76.47 d6 65.47 30.56 1.62 d11 3.70 13.07 16.46

[0107] In the observation optical systems shown in the first to ninth embodiments, because the magnification is varied by the objective lens, as compared with the conventional system in which the magnification is varied by the eyepiece lens, many advantages exist as follows.

[0108] (a) The real field coverage and the apparent field coverage in the wide-angle end can be widened.

[0109] (b) The change of the apparent field angle due to zooming can be diminished.

[0110] (c) Even if the observer has a deviated eyesight from 0 in diopter (because of the nearsightedness or farsightedness), the eyesight error during zooming can be removed.

[0111] d) The zoom ratio can be taken large.

[0112] As described above, according to the present embodiments, it is made possible to realize an observation optical system of widened angular coverage at the low magnification without having to increase the size of the apparatus. 

1. An observation optical system comprising, in order from an object side to an observation side, an objective optical system having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, an erect optical system, and an eyepiece optical system of positive refractive power for observing an object image formed through said objective optical system, wherein an air separation between said first lens unit and said second lens unit and an air separation between said second lens unit and said third lens unit are made variable to effect variation of magnification.
 2. An observation optical system according to claim 1 , wherein at least one of said first to third lens units constituting said objective optical system is displaced in directions perpendicular to an optical axis to compensate for an image shake.
 3. An observation optical system according to claim 1 , further comprising a variable angle prism, wherein an image shake is compensated for by varying an apex angle of said variable angle prism.
 4. An observation optical system according to claim 1 , satisfying the following condition: 0.7<βt<1.4 where βt is an overall lateral magnification in a telephoto end of said objective optical system excluding said first lens unit.
 5. An observation optical system according to claim 1 , wherein said first lens unit remains stationary during variation of magnification, and said second lens unit and said third lens unit move along an optical axis to effect variation of magnification.
 6. An observation optical system according to claim 5 , wherein, during variation of magnification from a wide-angle end to a telephoto end, said second lens unit moves from the object side to the observation side.
 7. An observation optical system according to claim 1 , wherein the object image is formed closer to the observation side than said erect optical system.
 8. An observation optical system according to claim 1 , wherein focusing is performed by moving said first lens unit along an optical axis.
 9. An observation optical system according to claim 1 , wherein each of said first to third lens units has a cemented lens composed of a negative lens and a positive lens cemented together.
 10. An observation optical system according to claim 9 , where said first lens unit consists of one cemented lens composed of, in order from the object side to the observation side, a positive lens and a negative lens cemented together.
 11. An observation optical system according to claim 9 , where said second lens unit consists of one cemented lens composed of, in order from the object side to the observation side, a negative lens and a positive lens cemented together.
 12. An observation optical system according to claim 9 , where said third lens unit consists of, in order from the object side to the observation side, one positive lens and one cemented lens composed of a negative lens and a positive lens cemented together.
 13. An observation optical system comprising, in order from an object side to an observation side, an objective optical system having a front lens unit of positive refractive power, a variator lens unit and a compensator lens unit, an erect optical system, and an eyepiece optical system of positive refractive power for observing an object image formed through said objective optical system, wherein the following condition is satisfied: 0.7<βt<1.4 where βt is an overall lateral magnification in a telephoto end of said objective optical unit excluding said front lens unit.
 14. An optical apparatus comprising an observation optical system according to one of claims 1 to 13 .
 15. A binocular comprising, in order from an object side to an observation side, a pair of objective optical systems each having a first lens unit of positive refractive power, a second lens unit of negative refractive power and a third lens unit of positive refractive power, a pair of erect optical systems, and a pair of eyepiece optical systems of positive refractive power for observing a pair of object images respectively formed through said pair of objective optical systems, wherein in each of said pair of objective optical systems, an air separation between said first lens unit and said second lens unit and an air separation between said second lens unit and said third lens unit are made variable to effect variation of magnification.
 16. A binocular according to claim 15 , wherein left and right optical axes are adjusted by moving said respective first lens units in directions perpendicular to the optical axes.
 17. A binocular according to claim 15 , wherein each of said pair of erect optical systems has an entering optical axis and an exiting optical axis deviating from each other, and wherein interpupillary adjustment is performed by rotating said pair of erect optical systems and said pair of eyepiece optical systems around respective optical axes, as rotation axes, of said pair of objective optical systems.
 18. A binocular according to claim 17 , wherein each of said pair of erect optical systems is a porro-II type prism, and wherein left and right porro-II type prisms are disposed such that apices of large prisms constituting the porro-II type prisms confront each other in an middle position in a width of adjustment for interpupillary adjustment.
 19. A binocular according to claim 17 , wherein each of said pair of erect optical system is composed of a combination of an auxiliary prism and a roof prism having an entering optical axis and an exiting optical axis deviating from each other, and wherein left and right roof prisms are so disposed as to satisfy the following condition: 45°<A<135° where A is an angle a line which connects optical axes of said pair of objective optical systems with each other makes with respect to a line which connects the optical axis of each of said pair of objective optical systems with an apex of a roof surface of each of said roof prisms in a middle position in a width of adjustment for interpupillary adjustment.
 20. A binocular comprising, in order from an object side to an observation side, a pair of objective optical systems of variable focal length, a pair of erect optical systems each having an entering optical axis and an exiting optical axis deviating from each other, and a pair of eyepiece optical systems, wherein interpupillary adjustment is performed by rotating said pair of erect optical systems and said pair of eyepiece optical systems around respective optical axes, as rotation axes, of said pair of objective optical systems. 